The DMV doesn't call it 'sales tax' they call it 'use tax' but it's the same thing. 8.75%+ or whatever your local sales tax rate is.
You can often negotiate with the seller to write down a lower number to save you some money. yea, it's illegal.
When I bought my box, I paid very little for it as far as the DMV paper work goes. Then I turned right around and bought the hard top, floor mats and and bunch of other stuff from the previous owner at really inflated prices.
The letter of the law was not violated, but the spirit of the law was tromped all over. Fortunately I bought it from a lawyer who didn't give a crap about the spirit of the law, just like me.